.TH IP\-L2TP 8 "19 Apr 2012" "iproute2" "Linux"
.SH "NAME"
ip-l2tp - L2TPv3 static unmanaged tunnel configuration
.SH "SYNOPSIS"
.sp
.ad l
.in +8
.ti -8
.B ip
.RI "[ " OPTIONS " ]"
.B l2tp
.RI " { " COMMAND " | "
.BR help " }"
.sp
.ti -8
.BR "ip l2tp add tunnel"
.br
.BI remote " ADDR " local " ADDR "
.br
.B tunnel_id
.IR ID
.B peer_tunnel_id
.IR ID
.br
.RB "[ " encap " { " ip " | " udp " } ]"
.br
.RB "[ " udp_sport
.IR PORT
.RB " ] [ " udp_dport
.IR PORT
.RB " ]"
.br
.RB "[ " udp_csum " { " on " | " off " } ]"
.br
.RB "[ " udp6_csum_tx " { " on " | " off " } ]"
.br
.RB "[ " udp6_csum_rx " { " on " | " off " } ]"
.br
.ti -8
.BR "ip l2tp add session"
.RB "[ " name
.IR NAME
.RB " ]"
.br
.B tunnel_id
.IR ID
.B session_id
.IR ID
.B peer_session_id
.IR ID
.br
.RB "[ " cookie
.IR HEXSTR
.RB " ] [ " peer_cookie
.IR HEXSTR
.RB " ]"
.br
.RB "[ " l2spec_type " { " none " | " default " } ]"
.br
.RB "[ " seq " { " none " | " send " | " recv " | " both " } ]"
.br
.ti -8
.BR "ip l2tp del tunnel"
.B tunnel_id
.IR ID
.br
.ti -8
.BR "ip l2tp del session"
.B tunnel_id
.IR ID
.B session_id
.IR ID
.br
.ti -8
.BR "ip l2tp show tunnel" " [ " tunnel_id
.IR ID " ]"
.br
.ti -8
.BR "ip l2tp show session" " [ " tunnel_id
.IR ID .B " ] ["
.B session_id
.IR ID " ]"
.br
.ti -8
.IR NAME " := "
.IR STRING
.ti -8
.IR ADDR " := { " IP_ADDRESS " |"
.BR any " }"
.ti -8
.IR PORT " := { " NUMBER " }"
.ti -8
.IR ID " := { " NUMBER " }"
.ti -8
.ti -8
.IR HEXSTR " := { 8 or 16 hex digits (4 / 8 bytes) }"
.SH DESCRIPTION
The
.B ip l2tp
commands are used to establish static, or so-called
.I unmanaged
L2TPv3 ethernet tunnels. For unmanaged tunnels, there is no L2TP
control protocol so no userspace daemon is required - tunnels are
manually created by issuing commands at a local system and at a remote
peer.
.PP
L2TPv3 is suitable for Layer-2 tunneling. Static tunnels are useful
to establish network links across IP networks when the tunnels are
fixed. L2TPv3 tunnels can carry data of more than one session. Each
session is identified by a session_id and its parent tunnel's
tunnel_id. A tunnel must be created before a session can be created in
the tunnel.
.PP
When creating an L2TP tunnel, the IP address of the remote peer is
specified, which can be either an IPv4 or IPv6 address. The local IP
address to be used to reach the peer must also be specified. This is
the address on which the local system will listen for and accept
received L2TP data packets from the peer.
.PP
L2TPv3 defines two packet encapsulation formats: UDP or IP. UDP
encapsulation is most common. IP encapsulation uses a dedicated IP
protocol value to carry L2TP data without the overhead of UDP. Use IP
encapsulation only when there are no NAT devices or firewalls in the
network path.
.PP
When an L2TPv3 ethernet session is created, a virtual network
interface is created for the session, which must then be configured
and brought up, just like any other network interface. When data is
passed through the interface, it is carried over the L2TP tunnel to
the peer. By configuring the system's routing tables or adding the
interface to a bridge, the L2TP interface is like a virtual wire
(pseudowire) connected to the peer.
.PP
Establishing an unmanaged L2TPv3 ethernet pseudowire involves manually
creating L2TP contexts on the local system and at the peer. Parameters
used at each site must correspond or no data will be passed. No
consistency checks are possible since there is no control protocol
used to establish unmanaged L2TP tunnels. Once the virtual network
interface of a given L2TP session is configured and enabled, data can
be transmitted, even if the peer isn't yet configured. If the peer
isn't configured, the L2TP data packets will be discarded by
the peer.
.PP
To establish an unmanaged L2TP tunnel, use
.B l2tp add tunnel
and
.B l2tp add session
commands described in this document. Then configure and enable the
tunnel's virtual network interface, as required.
.PP
Note that unmanaged tunnels carry only ethernet frames. If you need to
carry PPP traffic (L2TPv2) or your peer doesn't support unmanaged
L2TPv3 tunnels, you will need an L2TP server which implements the L2TP
control protocol. The L2TP control protocol allows dynamic L2TP
tunnels and sessions to be established and provides for detecting and
acting upon network failures.
.SS ip l2tp add tunnel - add a new tunnel
.TP
.BI tunnel_id " ID"
set the tunnel id, which is a 32-bit integer value. Uniquely
identifies the tunnel. The value used must match the peer_tunnel_id
value being used at the peer.
.TP
.BI peer_tunnel_id " ID"
set the peer tunnel id, which is a 32-bit integer value assigned to
the tunnel by the peer. The value used must match the tunnel_id value
being used at the peer.
.TP
.BI remote " ADDR"
set the IP address of the remote peer. May be specified as an IPv4
address or an IPv6 address.
.TP
.BI local " ADDR"
set the IP address of the local interface to be used for the
tunnel. This address must be the address of a local interface. May be
specified as an IPv4 address or an IPv6 address.
.TP
.BI encap " ENCAP"
set the encapsulation type of the tunnel.
.br
Valid values for encapsulation are:
.BR udp ", " ip "."
.TP
.BI udp_sport " PORT"
set the UDP source port to be used for the tunnel. Must be present
when udp encapsulation is selected. Ignored when ip encapsulation is
selected.
.TP
.BI udp_dport " PORT"
set the UDP destination port to be used for the tunnel. Must be
present when udp encapsulation is selected. Ignored when ip
encapsulation is selected.
.TP
.BI udp_csum " STATE"
(IPv4 only) control if IPv4 UDP checksums should be calculated and checked for the
encapsulating UDP packets, when UDP encapsulating is selected.
Default is
.BR off "."
.br
Valid values are:
.BR on ", " off "."
.TP
.BI udp6_csum_tx " STATE"
(IPv6 only) control if IPv6 UDP checksums should be calculated for encapsulating
UDP packets, when UDP encapsulating is selected.
Default is
.BR on "."
.br
Valid values are:
.BR on ", " off "."
.TP
.BI udp6_csum_rx " STATE"
(IPv6 only) control if IPv6 UDP checksums should be checked for the encapsulating
UDP packets, when UDP encapsulating is selected.
Default is
.BR on "."
.br
Valid values are:
.BR on ", " off "."
.SS ip l2tp del tunnel - destroy a tunnel
.TP
.BI tunnel_id " ID"
set the tunnel id of the tunnel to be deleted. All sessions within the
tunnel must be deleted first.
.SS ip l2tp show tunnel - show information about tunnels
.TP
.BI tunnel_id " ID"
set the tunnel id of the tunnel to be shown. If not specified,
information about all tunnels is printed.
.SS ip l2tp add session - add a new session to a tunnel
.TP
.BI name " NAME "
sets the session network interface name. Default is l2tpethN.
.TP
.BI tunnel_id " ID"
set the tunnel id, which is a 32-bit integer value. Uniquely
identifies the tunnel into which the session will be created. The
tunnel must already exist.
.TP
.BI session_id " ID"
set the session id, which is a 32-bit integer value. Uniquely
identifies the session being created. The value used must match the
peer_session_id value being used at the peer.
.TP
.BI peer_session_id " ID"
set the peer session id, which is a 32-bit integer value assigned to
the session by the peer. The value used must match the session_id
value being used at the peer.
.TP
.BI cookie " HEXSTR"
sets an optional cookie value to be assigned to the session. This is a
4 or 8 byte value, specified as 8 or 16 hex digits,
e.g. 014d3636deadbeef. The value must match the peer_cookie value set
at the peer. The cookie value is carried in L2TP data packets and is
checked for expected value at the peer. Default is to use no cookie.
.TP
.BI peer_cookie " HEXSTR"
sets an optional peer cookie value to be assigned to the session. This
is a 4 or 8 byte value, specified as 8 or 16 hex digits,
e.g. 014d3636deadbeef. The value must match the cookie value set at
the peer. It tells the local system what cookie value to expect to
find in received L2TP packets. Default is to use no cookie.
.TP
.BI l2spec_type " L2SPECTYPE"
set the layer2specific header type of the session.
.br
Valid values are:
.BR none ", " default "."
.TP
.BI seq " SEQ"
controls sequence numbering to prevent or detect out of order packets.
.B send
puts a sequence number in the default layer2specific header of each
outgoing packet.
.B recv
reorder packets if they are received out of order.
Default is
.BR none "."
.br
Valid values are:
.BR none ", " send ", " recv ", " both "."
.SS ip l2tp del session - destroy a session
.TP
.BI tunnel_id " ID"
set the tunnel id in which the session to be deleted is located.
.TP
.BI session_id " ID"
set the session id of the session to be deleted.
.SS ip l2tp show session - show information about sessions
.TP
.BI tunnel_id " ID"
set the tunnel id of the session(s) to be shown. If not specified,
information about sessions in all tunnels is printed.
.TP
.BI session_id " ID"
set the session id of the session to be shown. If not specified,
information about all sessions is printed.
.SH EXAMPLES
.PP
.SS Setup L2TP tunnels and sessions
.nf
site-A:# ip l2tp add tunnel tunnel_id 3000 peer_tunnel_id 4000 \\
           encap udp local 1.2.3.4 remote 5.6.7.8 \\
           udp_sport 5000 udp_dport 6000
site-A:# ip l2tp add session tunnel_id 3000 session_id 1000 \\
           peer_session_id 2000

site-B:# ip l2tp add tunnel tunnel_id 4000 peer_tunnel_id 3000 \\
           encap udp local 5.6.7.8 remote 1.2.3.4 \\
           udp_sport 6000 udp_dport 5000
site-B:# ip l2tp add session tunnel_id 4000 session_id 2000 \\
           peer_session_id 1000

site-A:# ip link set l2tpeth0 up mtu 1488

site-B:# ip link set l2tpeth0 up mtu 1488
.fi
.PP
Notice that the IP addresses, UDP ports and tunnel / session ids are
matched and reversed at each site.
.SS Configure as IP interfaces
The two interfaces can be configured with IP addresses if only IP data
is to be carried. This is perhaps the simplest configuration.
.PP
.nf
site-A:# ip addr add 10.42.1.1 peer 10.42.1.2 dev l2tpeth0

site-B:# ip addr add 10.42.1.2 peer 10.42.1.1 dev l2tpeth0

site-A:# ping 10.42.1.2
.fi
.PP
Now the link should be usable. Add static routes as needed to have
data sent over the new link.
.PP
.SS Configure as bridged interfaces
To carry non-IP data, the L2TP network interface is added to a bridge
instead of being assigned its own IP address, using standard Linux
utilities. Since raw ethernet frames are then carried inside the
tunnel, the MTU of the L2TP interfaces must be set to allow space for
those headers.
.PP
.nf
site-A:# ip link set l2tpeth0 up mtu 1446
site-A:# ip link add br0 type bridge
site-A:# ip link set l2tpeth0 master br0
site-A:# ip link set eth0 master br0
site-A:# ip link set br0 up
.fi
.PP
If you are using VLANs, setup a bridge per VLAN and bridge each VLAN
over a separate L2TP session. For example, to bridge VLAN ID 5 on eth1
over an L2TP pseudowire:
.PP
.nf
site-A:# ip link set l2tpeth0 up mtu 1446
site-A:# ip link add brvlan5 type bridge
site-A:# ip link set l2tpeth0.5 master brvlan5
site-A:# ip link set eth1.5 master brvlan5
site-A:# ip link set brvlan5 up
.fi
.PP
Adding the L2TP interface to a bridge causes the bridge to forward
traffic over the L2TP pseudowire just like it forwards over any other
interface. The bridge learns MAC addresses of hosts attached to each
interface and intelligently forwards frames from one bridge port to
another. IP addresses are not assigned to the l2tpethN interfaces. If
the bridge is correctly configured at both sides of the L2TP
pseudowire, it should be possible to reach hosts in the peer's bridged
network.
.PP
When raw ethernet frames are bridged across an L2TP tunnel, large
frames may be fragmented and forwarded as individual IP fragments to
the recipient, depending on the MTU of the physical interface used by
the tunnel. When the ethernet frames carry protocols which are
reassembled by the recipient, like IP, this isn't a problem. However,
such fragmentation can cause problems for protocols like PPPoE where
the recipient expects to receive ethernet frames exactly as
transmitted. In such cases, it is important that frames leaving the
tunnel are reassembled back into a single frame before being
forwarded on. To do so, enable netfilter connection tracking
(conntrack) or manually load the Linux netfilter defrag modules at
each tunnel endpoint.
.PP
.nf
site-A:# modprobe nf_defrag_ipv4

site-B:# modprobe nf_defrag_ipv4
.fi
.PP
If L2TP is being used over IPv6, use the IPv6 defrag module.
.SH INTEROPERABILITY
.PP
Unmanaged (static) L2TPv3 tunnels are supported by some network
equipment vendors such as Cisco.
.PP
In Linux, L2TP Hello messages are not supported in unmanaged
tunnels. Hello messages are used by L2TP clients and servers to detect
link failures in order to automate tearing down and reestablishing
dynamic tunnels. If a non-Linux peer supports Hello messages in
unmanaged tunnels, it must be turned off to interoperate with Linux.
.PP
Linux defaults to use the Default Layer2SpecificHeader type as defined
in the L2TPv3 protocol specification, RFC3931. This setting must be
consistent with that configured at the peer. Some vendor
implementations (e.g. Cisco) default to use a Layer2SpecificHeader
type of None.
.SH SEE ALSO
.br
.BR ip (8)
.SH AUTHOR
James Chapman <jchapman@katalix.com>
